Electro-optical device for the display of dark symbols composed of separately selectable display segments against a bright background and a means for addressing this device

Abstract

An electro-optical display for the display of dark symbols against a bright background, wherein except for fine separation lines the front and rear cell plates are almost entirely covered with isolated electrode elements. The regions of overlap between these various front and rear electrode elements define the areas of the selectable display segments. The display employs an electro-optic media requiring no polarizers, for example, cholesteric liquid crystals to which has been added pleochroic dyes, or electrophoretic suspensions. The display device is addressed through the application of appropriate signals to the electrodes so that a vanishingly small potential difference lies across the liquid crystal layer in the region of the selected display segments which remain dark. In all the other regions of the display, however, there exists a potential difference which is larger than the transition voltage of the electro-optical medium, and these regions therefore appear as a bright background. The addressing is preferably carried out using unipolar pulsed signals which are applied to the electrode elements of the front and rear cell plates, with those electrodes corresponding to optically selected display segments receiving signals having the same phase and amplitude.

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. In an electro-optical display device for the display of dark symbols against a bright background, wherein a plurality of separately selectable display segments are formed by a transparent front electrode applied to the surface of a transparent front cell plate and a rear electrode applied to the surface of a rear cell plate between which is located an electro-optical medium formed by a guest-host liquid crystal mixture, in particular a cholesteric liquid crystal with at least one added pleochroic dye, said cholesteric liquid crystal exhibiting a transition voltage where the cholesteric liquid crystal phase transforms to a nematic phase, said front electrode composed of a plurality of electrode elements that are electrically insulated from each other, said rear electrode composed of at least two electrode elements that cover nearly the entire area of the rear cell plate except for fine separation lines which serve to insulate them from each other, said front and said rear electrode elements having conduction paths leading to them, said front electrode having narrow separation lines which serve to electrically insulate the electrode elements thereof from each other and whose dimensions are small compared to the dimensions of the displayed symbols, said front electrode, except for said narrow separation lines, covering nearly the entire area of the front cell plate, and wherein at least one section of the separation line on the front electrode coincides with at least one third of the circumference of at least one display segment of a display symbol and at least one section of the separation line on the rear electrode coincides with at least one third of the circumference of the same display segment, improved means for driving the front and rear electrodes corresponding to an optically selected display element, comprising: means for addressing the display device whereby selected electrode segments are displayed against said bright background, said addressing means comprising,   first means for applying first unipolar addressing signals to the selected front and rear electrodes corresponding to the selected display segments of a desired symbol, said first addressing signal having the same amplitude, frequency, phase and waveform, and   second means for applying second addressing signals to the front and rear electrodes corresponding to unselected display segments, said second signals producing a potential difference in every region of the display background larger than the transition voltage of the electro-optical medium.   
     
     
       2. A display device according to claim 1 wherein said addressing means further comprises: said second signals applied to the electrodes on the front cell plate corresponding to optically unselected display elements as well as the rear electrode element which does not coincide with any of the display elements comprising a signal phase shifted by at most a half-period and having the same amplitude, frequency and waveform as said first addressing signals applied to the electrodes corresponding to the optically selected display segments; and,   said second signals applied to the electrode on the rear cell plate which does not overlap with any display segment comprising a signal which has a higher frequency than the other of said first and said second addressing signals.   
     
     
       3. A display device according to claim 2 wherein the frequency of said higher frequency second signals is a harmonic of those of said first signals applied to said selected electrodes. 
     
     
       4. A display device according to claim 1, further comprising: said first addressing signals comprising a square wave signal having an amplitude that is at least twice the transition voltage of the electro-optical medium;   said second addressing signals applied to the unselected front electrodes comprising a square wave signal having the same amplitude frequency and waveform as the said first addressing signals, but shifted in phase relative thereto by at most a half period; and   said second addressing signal applied to the electrode on the rear cell plate which does not overlap with any display segment comprising a d.c. voltage having half the amplitude of said first addressing signals.   
     
     
       5. A display device according to claim 4 wherein said first addressing signals have a frequency of at most 10 Hz. 
     
     
       6. A display device according to claim 3 wherein the high frequency second addressing signals applied to the electrode on the rear cell plate which does not overlap any display segment has a frequency in the range from 20 to 50 Hz. 
     
     
       7. A display device according to claim 3, wherein said high frequency second addressing signal applied to the electrode on the rear cell plate which does not overlap any display segment has an amplitude of approximately 10 volts and a frequency of approximately 30 Hz, and said transition voltage of said electro-optic medium is approximately 7.5 volts.